Diagrammatic method for modal syllogistic

I develop a diagrammatic method for Modal Syllogistic restricted to de dicto modalities in their contemporary interpretation. This method is a conservative extension of a previous diagrammatic method for Assertoric Syllogistic. It uses two types of inference rules: the intralevel inference rules contain the inference rules of the diagrammatic method for Assertoric Syllogistic, while the interlevel inference rules implement Theophrastus’ in peiorem rule.

Keywords:
Syllogism; modal logic; de dicto; diagrammatic method; Theophrastus; in peiorem